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c h a p t e r
2
The view taken here is that the life
cycle is the central unit in biology. …
Evolution then becomes the alteration
of life cycles through time, genetics the
inheritance mechanisms between cycles,
and development all the changes in
structure that take place during one life
cycle.
J. T. BONNER (1965)
It’s the circle of life
And it moves us all.
TIM RICE (1994)
Life cycles and the evolution
of developmental patterns
T
RADITIONAL WAYS OF CLASSIFYING ANIMALS catalog them according to their adult
structure. But, as J. T. Bonner (1965) pointed out, this is a very artificial
method, because what we consider an individual is usually just a brief slice of
its life cycle. When we consider a dog, for instance, we usually picture an adult. But
the dog is a “dog” from the moment of fertilization of a dog egg by a dog sperm. It
remains a dog even as a senescent dying hound. Therefore, the dog is actually the entire life cycle of the animal, from fertilization through death.
The life cycle has to be adapted to its environment, which is composed of nonliving objects as well as other life cycles. Take, for example, the life cycle of Clunio
marinus, a small fly that inhabits tidal waters along the coast of western Europe. Females of this species live only 2–3 hours as adults, and they must mate and lay their
eggs within this short time. To make matters even more precarious, they must lay
their eggs on red algal mats that are exposed only during the lowest ebbing of the
spring tide. Such low tides occur on four successive days shortly after the new and
full moons (i.e., at about 15-day intervals). Therefore, the life cycle of these insects
must be coordinated with the lunar cycle as well as the daily tidal rhythms such that
the insects emerge from their pupal cases during the few days of the spring tide and
at the correct hour for its ebb (Beck 1980; Neumann and Spindler 1991).
The Circle of Life: The Stages of Animal Development
One of the major triumphs of descriptive embryology was the idea of a generalizable
life cycle. Each animal, whether earthworm, eagle, or beagle, passes through similar
stages of development. The life of a new individual is initiated by the fusion of genetic material from the two gametes—the sperm and the egg. This fusion, called fertilization, stimulates the egg to begin development. The stages of development between fertilization and hatching are collectively called embryogenesis. Throughout
the animal kingdom, an incredible variety of embryonic types exist, but most patterns of embryogenesis are variations on five themes:
1. Immediately following fertilization, cleavage occurs. Cleavage is a series of extremely rapid mitotic divisions wherein the enormous volume of zygote cytoplasm is divided into numerous smaller cells. These cells are called blastomeres,
and by the end of cleavage, they generally form a sphere known as a blastula.
2. After the rate of mitotic division has slowed down, the blastomeres undergo dramatic movements wherein they change their positions relative to one another.
This series of extensive cell rearrangements is called gastrulation, and the embryo is said to be in the gastrula stage. As a result of gastrulation, the embryo
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Chapter 2
contains three germ layers: the ectoderm, the endoderm,
and the mesoderm.
3. Once the three germ layers are established, the cells interact with one another and rearrange themselves to produce tissues and organs. This process is called organogenesis. Many organs contain cells from more than one
germ layer, and it is not unusual for the outside of an
organ to be derived from one layer and the inside from
another. For example, the outer layer of skin (epidermis)
comes from the ectoderm, while the inner layer (the dermis) comes from the mesoderm. Also during organogenesis, certain cells undergo long migrations from their
place of origin to their final location. These migrating
cells include the precursors of blood cells, lymph cells,
pigment cells, and gametes. Most of the bones of our face
are derived from cells that have migrated ventrally from
the dorsal region of the head.
4. In many species, a specialized portion of egg cytoplasm
gives rise to cells that are the precursors of the gametes
(sperm and egg). The gametes and their precursor cells
are collectively called germ cells, and they are set aside
for reproductive function. All the other cells of the body
are called somatic cells. This separation of somatic cells
(which give rise to the individual body) and germ cells
(which contribute to the formation of a new generation)
is often one of the first differentiations to occur during
animal development. The germ cells eventually migrate
to the gonads, where they differentiate into gametes. The
development of gametes, called gametogenesis, is usually
not completed until the organism has become physically
mature. At maturity, the gametes may be released and
participate in fertilization to begin a new embryo. The
adult organism eventually undergoes senescence and dies.
5. In many species, the organism that hatches from the egg or
is born into the world is not sexually mature. Indeed, in
most animals, the young organism is a larva that may look
significantly different from the adult. Larvae often constitute the stage of life that is used for feeding or dispersal. In
many species, the larval stage is the one that lasts the
longest, and the adult is a brief stage solely for reproduction. In the silkworm moths, for instance, the adults do not
have mouthparts and cannot feed. The larvae must eat
enough for the adult to survive and mate. Indeed, most female moths mate as soon as they eclose from their pupa,
and they fly only once—to lay their eggs. Then they die.
The Frog Life Cycle
Figure 2.1 uses the development of the leopard frog, Rana pipiens, to show a representative life cycle. Let us look at this life
cycle in a bit more detail.
In most frogs, gametogenesis and fertilization are seasonal
events, because its life depends on the plants and insects in the
2ND PASS PAGE PROOFS
pond where it lives and on the temperature of the air and
water. A combination of photoperiod (hours of daylight) and
temperature tells the pituitary gland of the female frog that it is
spring. If the female is mature, her pituitary gland secretes hormones that stimulate her ovary to make estrogen. Estrogen is a
hormone that can instruct the liver to make and secrete yolk
proteins such as vitellogenin, which are then transported
through the blood into the enlarging eggs in the ovary.* The
yolk is transported into the bottom portion of the egg (Figure
2.2A). The bottom half of the egg usually contains more yolk
than the top half and is called the vegetal hemisphere of the
egg. Conversely, the upper half of the egg usually has less yolk
and is called the animal hemisphere of the egg.†
Another ovarian hormone, progesterone, signals the egg
to resume its meiotic division. This is necessary because the
egg had been “frozen” in the metaphase of its first meiosis.
When it has completed this first meiotic division, the egg is
released from the ovary and can be fertilized. In many species,
the eggs are enclosed in a jelly coat that acts to enhance their
size (so they won’t be as easily eaten), to protect them against
bacteria, and to attract and activate sperm.
Sperm also occur on a seasonal basis. Male leopard frogs
make their sperm in the summer, and by the time they begin
hibernation in autumn, they have produced all the sperm that
are to be available for the following spring’s breeding season.
In most species of frogs, fertilization is external. The male frog
grabs the female’s back and fertilizes the eggs as the female
frog releases them (Figure 2.2B). Rana pipiens usually lays
about 2500 eggs, while the bullfrog, Rana catesbiana, can lay
as many as 20,000. Some species lay their eggs in pond vegetation, and the jelly adheres to the plants and anchors the eggs
(Figure 2.2C). Other species float their eggs into the center of
the pond without any support.
Fertilization accomplishes several things. First, it allows
the egg to complete its second meiotic division, which provides the egg with a haploid pronucleus. The egg pronucleus
and the sperm pronucleus meet in the egg cytoplasm to form
the diploid zygote nucleus. Second, fertilization causes the cytoplasm of the egg to move such that different parts of the cytoplasm find themselves in new locations (Figure 2.2D).
Third, fertilization activates those molecules necessary to
begin cell cleavage and development (Rugh 1950). The sperm
and egg die quickly unless fertilization occurs.
*As we will see in later chapters, there are numerous ways by which the synthesis of a new protein can be induced. Estrogen stimulates the production of vitellogenin protein in two ways. First, it uses transcriptional regulation to make new
vitellogenin mRNA. Before estrogen stimulation, no vitellogenin message can be
seen in the liver cells. After stimulation, there are over 50,000 vitellogenin mRNA
molecules in these cells. Estrogen also uses translational regulation to stabilize
these particular messages, increasing their half-life from 16 hours to 3 weeks. In
this way, more protein can be translated from each message.
†
The terms animal and vegetal reflect the movements of cells seen in some embryos (such as those of frogs). The cells derived from the upper portion of the
egg divide more rapidly and are actively mobile (hence, animated), while the
yolk-filled cells of the vegetal half were seen as being immobile (hence, like
plants).
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Life cycles and the evolution of developmental patterns
27
Sperm
Morula
Sperm
(male gamete)
Oocyte
(female gamete)
FERTILIZATION
Blastula
Location of
germ cells
Oocyte
Germ plasm
Blastocoel
CLEAVAGE
Blastopore
GAMETOGENESIS
GASTRULATION
Sexually
mature
adult
Gonad
Ectoderm
MATURITY
Mesoderm
Metamorphosis
(in some species)
ORGANOGENESIS
Endoderm
LARVAL
STAGES
Immature larval
stages
Hatching
(birth)
Figure 2.1
Developmental history of the leopard frog, Rana pipiens. The stages
from fertilization through hatching (birth) are known collectively as
embryogenesis. The region set aside for producing germ cells is
shown in bright purple. Gametogenesis, which is completed in the
sexually mature adult, begins at different times during development,
depending on the species. (The sizes of the varicolored wedges
shown here are arbitrary and do not correspond to the proportion
of the life cycle spent in each stage.)
During cleavage, the volume of the frog egg stays the
same, but it is divided into tens of thousands of cells (Figure
2.2E–H). The animal hemisphere of the egg divides faster
than the vegetal hemisphere does, and the cells of the vegetal
hemisphere become progressively larger the more vegetal the
cytoplasm. A fluid-filled cavity, the blastocoel, forms in the
animal hemisphere (Figure 2.2I). This cavity will be important for allowing cell movements to occur during gastrulation.
Gastrulation in the frog begins at a point on the embryo
surface roughly 180 degrees opposite the point of sperm entry
with the formation of a dimple, called the blastopore. At first,
just a small slit is made. Cells migrating through this dorsal
blastage lip migrate toward the animal pole (Figure 2.3A,B).
These cells become the dorsal mesoderm. The blastopore expands into a circle (Figure 2.3C), and cells migrating through
the lateral and ventral lips of this circle become the lateral and
ventral mesoderm. The cells remaining on the outside become
the ectoderm, and this outer layer expands vegetally to enclose
the entire embryo. The large yolky cells that remain at the veg-
etal hemisphere (until they are encircled by the ectoderm) become the endoderm. Thus, at the end of gastrulation, the ectoderm (the precursor of the epidermis and nerves) is on the
outside of the embryo, the endoderm (the precursor of the
gut lining) is on the inside of the embryo, and the mesoderm
(the precursor of connective tissue, blood, skeleton, gonads,
and kidneys) is between them.
Organogenesis begins when the notochord—a rod of
mesodermal cells in the most dorsal portion of the embryo—
signals the ectodermal cells above it that they are not going to
become epidermis. Instead, these dorsal ectoderm cells form a
tube and become the nervous system. At this stage, the embryo is called a neurula. The neural precursor cells elongate,
stretch, and fold into the embryo (Figure 2.3D–F), forming
the neural tube. The future back epidermal cells cover them.
The cells that had connected the neural tube to the epidermis
become the neural crest cells. The neural crest cells are almost
like a fourth germ layer. They give rise to the pigment cells of
the body (the melanocytes), the peripheral neurons, and the
cartilage of the face.
Once the neural tube has formed, it induces changes in
its neighbors, and organogenesis continues. The mesodermal
tissue adjacent to the notochord becomes segmented into
somites, the precursors of the frog’s back muscles, spinal vertebrae, and dermis (the inner portion of the skin). These
somites appear as blocks of mesodermal tissue (Figure
2.3F,G). The embryo develops a mouth and an anus, and it
elongates into the typical tadpole structure (Figure 2.3H). The
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Chapter 2
Figure 2.2
Early development of the frog Xenopus laevis. (A) As the egg matures, it accumulates
yolk (here stained yellow and green) in the vegetal cytoplasm. (B) Frogs mate by amplexus, the male grasping the female around the belly and fertilizing the eggs as they
are released. (C) A newly laid clutch of eggs. The brown area of each egg is the pigmented animal cap. The white spot in the middle of the pigment is where the egg's
nucleus resides. (D) Rearrangement of cytoplasm seen during first cleavage. Compare
with the initial stage seen in (A). (E) A 2-cell embryo near the end of its first cleavage.
(F) An 8-cell embryo. (G) Early blastula. Note that the cells get smaller, but the volume of the egg remains the same. (H) Late blastula. (I) Cross section of a late blastula,
showing the blastocoel (cavity). (A–H courtesy of Michael Danilchik and Kimberly
Ray; I courtesy of J. Heasman.)
(A)
(B)
(C)
(D)
(E)
(F)
(G)
(H)
(I)
Blastocoel
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Life cycles and the evolution of developmental patterns
29
Figure 2.3
(A)
Continued development of Xenopus laevis. (A) Gastrulation begins with an invagination,
or slit, in the future dorsal (top) side of the embryo. (B) This slit, the dorsal blastopore
lip, as seen from the ventral surface (bottom) of the embryo. (C) The slit becomes a circle, the blastopore. Future mesoderm cells migrate into the interior of the embryo along
the blastopore edges, and the ectoderm (future epidermis and nerves) migrates down the
outside of the embryo. The remaining part, the yolk-filled endoderm, is eventually encircled. (D) Neural folds begin to form on the dorsal surface. (E) A groove can be seen
where the bottom of the neural tube will be. (F) The neural folds come together at the
dorsal midline, creating a neural tube. (G) Cross section of the Xenopus embryo at the
neurula stage. (H) A pre-hatching tadpole, as the protrusions of the forebrain begin to induce eyes to form. (I) A mature tadpole, having swum away from the egg mass and feeding independently. (Photographs courtesy of Michael Danilchik and Kimberly Ray.)
(B)
Dorsal blastopore lip
(C) Yolk plug
Blastopore
(D)
(F)
(E)
Dorsal blastopore lip
(G)
Dorsal
(back)
Notochord
Neural folds
Neural tube
Somite
Archenteron
(future gut)
Yolky endoderm
Epidermis
(ectoderm)
Open neural tube
Neural groove
(H)
(I)
Somites
Brain
Gill area
Expansion of
forebrain to touch
surface ectoderm
((induces eyes to form)
Stomodeum (mouth)
Tailbud
Mesoderm
Ventral
(belly)
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Chapter 2
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(B)
(A)
(C)
(D)
(E)
Figure 2.4
Metamorphosis of the frog. (A) Huge changes are obvious when one contrasts the tadpole
and the adult bullfrog. Note especially the differences in jaw structure and limbs. (B) Premetamorphic tadpole. (C) Prometamorphic tadpole, showing hindlimb growth. (D) Onset
of metamorphic climax as forelimbs emerge. (E, F) Climax stages. (Photograph copyright
Patrice Ceisel/Stock, Boston.)
neurons make their connections to the muscles and to other
neurons, the gills form, and the larva is ready to hatch from its
egg jelly. The hatched tadpole will soon feed for itself once the
yolk supply given it by its mother is exhausted (Figure 2.3I).
Metamorphosis of the tadpole larva into an adult frog is
one of the most striking transformations in all of biology (Figure 2.4). These changes prepare an aquatic organism for a terrestrial existence. In amphibians, metamorphosis is initiated by
hormones from the tadpole’s thyroid gland. (The mechanisms
by which thyroid hormones accomplish these changes will be
discussed in Chapter 18.) In anurans (frogs and toads), almost
every organ is subject to modification, and the resulting
changes in form are striking and very obvious. The hindlimbs
and forelimbs that the adult will use for locomotion differentiate and the tadpole’s paddle tail recedes. The cartilaginous tadpole skull is replaced by the predominantly bony skull of the
young frog. The horny teeth the tadpole uses to tear up pond
plants disappear as the mouth and jaw take a new shape, and
the fly-catching tongue muscle of the frog develops. Meanwhile, the large intestine characteristic of herbivores shortens
to suit the more carnivorous diet of the adult frog. The gills
regress and the lungs enlarge. As metamorphosis ends, the development of the first germ cells begins. In Rana pipiens, egg
development lasts 3 years. At that time, the female frog is sexually mature and can produce offspring of her own.
The speed of metamorphosis is carefully keyed to environmental pressures. In temperate regions, for instance, Rana
metamorphosis must occur before ponds freeze in winter. An
adult leopard frog can burrow into the mud and survive the
winter; its tadpole cannot.
(F)
WEBSITE 2.1 Immortal animals. Imagine a multicellu-
lar animal that acquires immortality by reverting to its larval form instead of growing old. That seems to be what the
marine hydranth Turritopsis does.
WEBSITE 2.2 The human life cycle. The human animal
provides a fascinating life cycle to study. Here are some
websites that speculate about (A) when is an embryo or
fetus “human”? (B) how might the strange way the human
brain develops make childhood a necessity? and (C) do humans undergo metamorphosis?
VADE MECUM The frog life cycle. The life cycle of a frog
is illustrated in labeled photographs and time-lapse
videomicroscopy. [Click on Amphibian]
The Evolution of Developmental Patterns
in Unicellular Protists
Every living organism develops. Development can be seen
even among the unicellular organisms. Moreover, by studying
the development of unicellular protists, we can see the simplest forms of cell differentiation and sexual reproduction.
Control of developmental morphogenesis: The role
of the nucleus
A century ago, it had not yet been proved that the nucleus of
the cell contained hereditary or developmental information.
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Some of the best evidence for this theory came from studies
in which unicellular organisms were fragmented into nucleate
and anucleate pieces (reviewed in Wilson 1896). When various protists were cut into fragments, nearly all the pieces died.
However, the fragments containing nuclei were able to live
and to regenerate entire complex cellular structures.
Nuclear control of cell morphogenesis and the interaction of nucleus and cytoplasm are beautifully demonstrated in
studies of the protist Acetabularia. This enormous single cell
(2–4 cm long) consists of three parts: a cap, a stalk, and a rhizoid (Figure 2.5A; Mandoli 1998). The rhizoid is located at
the base of the cell and holds it to the substrate. The single
nucleus of the cell resides within the rhizoid. The size of Acetabularia and the location of its nucleus allow investigators to
remove the nucleus from one cell and replace it with a nucleus
from another cell. In the 1930s, J. Hämmerling took advantage
of these unique features and exchanged nuclei between two
morphologically distinct species, A. mediterranea* and A.
crenulata. As Figure 2.5A shows, these two species have very
different cap structures. Hämmerling found that when he
transferred the nucleus from one species into the stalk of another species, the newly formed cap eventually assumed the
form associated with the donor nucleus (Figure 2.5B). Thus,
the nucleus was seen to control Acetabularia development.
The formation of a cap is a complex morphogenic event
involving the synthesis of numerous proteins, which must be
accumulated in a certain portion of the cell and then assembled into complex, species-specific structures. The transplanted nucleus does indeed direct the synthesis of its species*After a recent formal name change, this species is now called Acetabularia acetabulum. For the sake of simplicity, however, we will use Hämmerling’s designations here.
Life cycles and the evolution of developmental patterns
specific cap, but it takes several weeks to do so. Moreover, if the
nucleus is removed from an Acetabularia cell early in its development, before it first forms a cap, a normal cap is formed
weeks later, even though the organism will eventually die.
These studies suggest that (1) the nucleus contains information specifying the type of cap to be produced (i.e., it contains
the genetic information that specifies the proteins required for
the production of a certain type of cap), and (2) material containing this information enters the cytoplasm long before cap
production occurs. This information in the cytoplasm is not
used for several weeks.
One current hypothesis proposed to explain these observations is that the nucleus synthesizes a stable mRNA that lies dormant in the cytoplasm until the time of cap formation (see Dumais et al. 2000). This hypothesis is supported by an observation
that Hämmerling published in 1934. Hämmerling fractionated
young Acetabularia into several parts (Figure 2.6). The portion
with the nucleus eventually formed a new cap, as expected; so
did the apical tip of the stalk. However, the intermediate portion
of the stalk did not form a
cap. Thus, Hämmerling pos(B)
tulated (nearly 30 years before the existence of mRNA
was known) that the instructions for cap formation
originated in the nucleus
and were somehow stored in
a dormant form near the tip
of the stalk. Many years later,
researchers established that
nucleus-derived mRNA does
accumulate in the tip of the
A. crenulata
A. mediterranea
Nuclei transplanted
(A)
Nucleus
Nucleus
Rhizoid
Figure 2.5
(A) Acetabularia crenulata (left)
and A. mediterranea (right). Each
individual is a single cell. The
rhizoid contains the nucleus.
(B) Effect of exchanging nuclei
between two species of Acetabularia. Nuclei were transplanted
into enucleated rhizoid fragments. A. crenulata structures are
darker, A. mediterranea structures lighter green. (Photographs
courtesy of S. Berger.)
31
Cap structure
is that of
donor nucleus
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Chapter 2
Cap and stalk
regenerated
Apical tip
of stalk
sential role in cap formation. The mRNAs are not translated
for weeks, even though they are present in the cytoplasm.
Something in the cytoplasm controls when the message is utilized. Hence, the expression of the cap is controlled not only
by nuclear transcription, but also by the translation of the cytoplasmic RNA. In this unicellular organism, “development” is
controlled at both the transcriptional and translational levels.
WEBSITE 2.3 Protist differentiation. Three of the most
Central portion
of stalk
remarkable areas of protist development concern the control of sex type in fission yeast, the transformation of
Naegleria amoebae into streamlined, flagellated cells, and
the cortical inheritance of the cell surface in paramecia.
No regeneration
Rhizoid
and nucleus
Total
regeneration
Figure 2.6
Regenerative ability of different fragments of
A. mediterranea.
stalk, and that the destruction of this mRNA or the inhibition
of protein synthesis in this region prevents cap formation
(Kloppstech and Schweiger 1975; Garcia and Dazy 1986;
Serikawa et al. 2001).
It is clear from the preceding discussion that nuclear
transcription plays an important role in the formation of the
Acetabularia cap. But note that the cytoplasm also plays an es-
Cytoplasmic bridge
Micronuclei
Macronucleus
Two paramecia form
cytoplasmic bridge
Unicellular protists and the origins
of sexual reproduction
Sexual reproduction is another invention of the protists that
has had a profound effect on more complex organisms. It
should be noted that sex and reproduction are two distinct
and separable processes. Reproduction involves the creation
of new individuals. Sex involves the combining of genes from
two different individuals into new arrangements. Reproduction in the absence of sex is characteristic of organisms that
reproduce by fission (i.e., splitting into two); there is no sorting of genes when an amoeba divides or when a hydra buds
off cells to form a new colony.
Sex without reproduction is also common among unicellular organisms. Bacteria are able to transmit genes from one
individual to another by means of sex pili. This transmission is
separate from reproduction. Protists are also able to reassort
genes without reproduction. Paramecia, for instance, reproduce by fission, but sex is accomplished by conjugation. When
two paramecia join together, they link their oral apparatuses
and form a cytoplasmic connection through which they can
Stationary
micronucleus
Meiotic
spindle
Micronuclei undergo
meiosis, forming
8 haploid nuclei
per cell; macronuclei
degenerate
All but one of
each partner’s
micronuclei
degenerate
Figure 2.7
Conjugation across a cytoplasmic bridge in paramecia. Two paramecia can
exchange genetic material, each ending up with genes that differ from those
with which they started. (After Strickberger 1985.)
Migratory
micronucleus
Remaining micronucleus divides to
form a stationary
and a migratory
micronucleus
Diploid nucleus forms and
undergoes mitotic divisions to
generate a new macronucleus
as paramecia separate
Migratory micronuclei
cross cytoplasmic bridge
and fertilize partners’
stationary micronuclei
2ND PASS PAGE PROOFS
exchange genetic material (Figure 2.7). The macronucleus of
each individual (which controls the metabolism of the organism) degenerates, while each micronucleus undergoes meiosis
to produce eight haploid micronuclei, of which all but one degenerate. The remaining micronucleus divides once more to
form a stationary micronucleus and a migratory micronucleus.
Each migratory micronucleus crosses the cytoplasmic bridge
and fuses with (“fertilizes”) the partner’s stationary micronucleus, thereby creating a new diploid nucleus in each cell. This
diploid nucleus then divides mitotically to give rise to a new
micronucleus and a new macronucleus as the two partners disengage. Therefore, no reproduction has occurred, only sex.
The union of these two distinct processes, sex and reproduction, into sexual reproduction is seen in other unicellular
eukaryotes. Figure 2.8 shows the life cycle of Chlamydomonas.
This organism is usually haploid, having just one copy of each
chromosome (like a mammalian gamete). The individuals of
each species, however, are divided into two mating types: plus
Asexual (mitotic) reproduction
Minus mating type
Plus mating type
(haploid)
(haploid)
Sexual
reproduction
Mating
Cytoplasms merge
Zygote (diploid)
Maturation (meiosis)
Germination
Two plus and two minus mating types
Figure 2.8
Sexual reproduction in Chlamydomonas. Two mating types, both
haploid, can reproduce asexually when separate. Under certain conditions, the two mating types can unite to produce a diploid cell
that can undergo meiosis to form four new haploid organisms.
(After Strickberger 1985.)
Life cycles and the evolution of developmental patterns
33
and minus. When a plus and a minus meet, they join their cytoplasms, and their nuclei fuse to form a diploid zygote. This
zygote is the only diploid cell in the life cycle, and it eventually
undergoes meiosis to form four new Chlamydomonas cells.
This is true sexual reproduction, for chromosomes are reassorted during the meiotic divisions and more individuals are
formed. Note that in this protist type of sexual reproduction,
the gametes are morphologically identical; the distinction between sperm and egg has not yet been made.
In evolving sexual reproduction, two important advances
had to be achieved. The first was the mechanism of meiosis
(Figure 2.9), whereby the diploid complement of chromosomes is reduced to the haploid state (discussed in detail in
Chapter 19). The second was a mechanism whereby the two
different mating types could recognize each other. In Chlamydomonas, recognition occurs first on the flagellar membranes
(Figure 2.10; Bergman et al. 1975; Wilson et al. 1997; Pan and
Snell 2000). The flagella of two individuals twist around each
other, enabling specific regions of the cell membranes to come
together. These specialized regions contain mating type-specific components that enable the cytoplasms to fuse. Following flagellar agglutination, the plus individual initiates fusion
by extending a fertilization tube. This tube contacts and fuses
with a specific site on the minus individual. Interestingly, the
mechanism used to extend this tube—the polymerization of
the protein actin to form microfilaments—is also used to extend the processes of sea urchin eggs and sperm. In Chapter 7,
we will see that the recognition and fusion of sperm and egg
occur in an amazingly similar manner.
Unicellular eukaryotes appear to possess the basic elements of the developmental processes that characterize more
complex organisms: protein synthesis is controlled such that
certain proteins are made only at certain times and in certain
places; the structures of individual genes and chromosomes
are as they will be throughout eukaryotic evolution; mitosis
and meiosis have been perfected; and sexual reproduction exists, involving cooperation between individual cells. Such intercellular cooperation becomes even more important with
the evolution of multicellular organisms.
Multicellularity:
The Evolution of Differentiation
One of evolution’s most important experiments was the creation of multicellular organisms. There appear to be several
paths by which single cells evolved multicellular arrangements; we will discuss only two of them here (see Chapter 22
for a fuller discussion). The first path involves the orderly division of the reproductive cell and the subsequent differentiation of its progeny into different cell types. This path to multicellularity can be seen in a remarkable series of multicellular
organisms collectively referred to as the family Volvocaceae, or
the volvocaceans (Kirk 1999, 2000).
34
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Chapter 2
Meiosis I: Separation of homologous chromosomes
Nuclear
envelope
Chromatin
Homologous
chromosomes
Homologous
chromatids
Nucleus
Interphase
DNA replicates
Early prophase I
Mid prophase I
Late prophase I
Metaphase I
The nuclear envelope breaks down and homologous chromosomes
(each chromosome being double, with the chromatids joined at the
kinetochore) align in pairs. Chromosomal rearrangements can occur
between the four homologous chromatids at this time
Figure 2.9
Summary of meiosis. The DNA and its associated proteins replicate
during interphase. During prophase, the nuclear envelope breaks
down and homologous chromosomes (each chromosome is double,
with the chromatids joined at the kinetochore) align in pairs.
Chromosomal rearrangements between the four homologous chromatids can occur at this time. After the first metaphase, the two
original homologous chromosomes are segregated into different
cells. During the second meiotic division, the kinetochore splits and
the sister chromatids separate, leaving each new cell with one copy
of each chromosome.
The Volvocaceans
The simpler organisms among the volvocaceans are ordered
assemblies of numerous cells, each resembling the unicellular
protist Chlamydomonas, to which they are related (Figure
2.11A). A single organism of the volvocacean genus Gonium
(Figure 2.11B), for example, consists of a flat plate of 4 to 16
cells, each with its own flagellum. In a related genus, Pandorina, the 16 cells form a sphere (Figure 2.11C); and in Eudorina, the sphere contains 32 or 64 cells arranged in a regular
pattern (Figure 2.11D). In these organisms, then, a very important developmental principle has been worked
out: the ordered division of one cell to generate
(A)
(B)
a number of cells that are organized in a predictable fashion. Like cleavage in most animal
embryos, the cell divisions by which a single
volvocacean cell produces an organism of 4 to
64 cells occur in very rapid sequence and in the
absence of cell growth.
The next two genera of the volvocacean series
exhibit another important principle of development: the differentiation of cell types within an
individual organism. In these organisms, the reproductive cells become differentiated from the
somatic cells. In all the genera mentioned earlier,
Microfilaments
every cell can, and normally does, produce a
complete new organism by mitosis. In the genera
Pleodorina and Volvox, however, relatively few
cells can reproduce. In Pleodorina californica (Figure 2.11E), the cells in the anterior region are restricted to a somatic function; only those cells on
the posterior side can reproduce. In P. californica,
Figure 2.10
Two-step recognition in mating Chlamydomonas. (A) Scanning electron microa colony usually has 128 or 64 cells, and the ratio
graph (7000×) of mating pair. The interacting flagella twist around each other, adof the number of somatic cells to the number of
hering at the tips (arrows). (B) Transmission electron micrograph (20,000×) of a
reproductive cells is usually 3:5. Thus, a 128-cell
cytoplasmic bridge connecting the two organisms. The actin microfilaments extend
colony typically has 48 somatic cells, and a 64-cell
from the donor (lower) cell to the recipient (upper) cell. (From Goodenough and
colony has 24.
Weiss 1975 and Bergman et al. 1975; photographs courtesy of U. Goodenough.)
2ND PASS PAGE PROOFS
Life cycles and the evolution of developmental patterns
35
Meiosis II: Separation of the chromatids
Anaphase I
Telophase I
Metaphase II
The two original homologous chromosomes
are segregated into
different cells
In Volvox, almost all the cells are somatic, and very few of
the cells are able to produce new individuals. In some species
of Volvox, reproductive cells, as in Pleodorina, are derived from
cells that originally look and function like somatic cells before
they enlarge and divide to form new progeny. However, in
other members of the genus, such as V. carteri, there is a complete division of labor: the reproductive cells that will create
the next generation are set aside during the division of the
original cell that is forming a new individual. The reproductive cells never develop functional flagella and never contribute to motility or other somatic functions of the individual; they are entirely specialized for reproduction.
Thus, although the simpler volvocaceans may be thought
of as colonial organisms (because each cell is capable of inde(A)
(D)
(B)
(E)
Anaphase II
Telophase II
The kinetochore splits
Each new cell has
one copy of each
chromosome
pendent existence and of perpetuating the species), in V. carteri we have a truly multicellular organism with two distinct
and interdependent cell types (somatic and reproductive),
both of which are required for perpetuation of the species
(Figure 2.11F). Although not all animals set aside the reproductive cells from the somatic cells (and plants hardly ever
do), this separation of germ cells from somatic cells early in
development is characteristic of many animal phyla and will
be discussed in more detail in Chapter 19.
WEBSITE 2.4 Volvox cell differentiation. The pathways
leading to germ cells or somatic cells are controlled by genes
that cause cells to follow one or the other fate. Mutations
can prevent the formation of one of these lineages.
(C)
(F)
Figure 2.11
Representatives of the order Volvocales.
All but Chlamydomonas are members of
the family Volvocaceae. (A) The unicellular protist Chlamydomonas reinhardtii.
(B) Gonium pectorale, with 8 Chlamydomonas-like cells in a convex disc.
(C) Pandorina morum. (D) Eudorina elegans. (E) Pleodorina californica. Here,
all 64 cells are originally similar, but the
posterior ones dedifferentiate and redifferentiate as asexual reproductive cells
called gonidia, while the anterior cells
remain small and biflagellate, like
Chlamydomonas. (F) Volvox carteri.
Here, cells destined to become gonidia
are set aside early in development and
never have somatic characteristics. The
smaller somatic cells resemble Chlamydomonas. Complexity increases from
the single-celled Chlamydomonas to the
multicellular Volvox. (Photographs
courtesy of D. Kirk.)
36
2ND PASS PAGE PROOFS
Chapter 2
Sidelights
Speculations
Sex and Individuality in Volvox
S
imple as it is, Volvox shares many features that characterize the life cycles
and developmental histories of much
more complex organisms, including ourselves. As already mentioned, Volvox is
among the simplest organisms to exhibit a
division of labor between two completely
different cell types. As a consequence, it is
among the simplest organisms to include
death as a regular, genetically regulated
part of its life history.
Death and Differentiation
Unicellular organisms that reproduce by
simple cell division, such as amoebae, are
potentially immortal. The amoeba you see
today under the microscope has no dead
ancestors. When an amoeba divides, neither of the two resulting cells can be considered either ancestor or offspring; they
are siblings. Death comes to an amoeba
only if it is eaten or meets with a fatal accident, and when it does, the dead cell
leaves no offspring.
Death becomes an essential part of life,
however, for any multicellular organism
that establishes a division of labor between
somatic (body) cells and germ (reproductive) cells. Consider the life history of
Volvox carteri when it is reproducing asexually (Figure 2.12). Each asexual adult is a
spheroid containing some 2000 small, bifla-
gellated somatic cells along its periphery
and about 16 large, asexual reproductive
cells, called gonidia, toward one end of the
interior. When mature, each gonidium divides rapidly 11 or 12 times. Certain of
these divisions are asymmetrical and produce the 16 large cells that will become a
new set of gonidia in the next generation.
At the end of cleavage, all the cells that
will be present in an adult have been produced from the gonidium. But the resulting embryo is “inside out”: it is now a hollow sphere with its gonidia on the outside
and the flagella of its somatic cells pointing
toward the interior. This predicament is
corrected by a process called inversion, in
which the embryo turns itself right side out
by a set of cell movements that resemble
the gastrulation movements of animal embryos (Figure 2.13A–H). Clusters of bottleshaped cells open a hole at one end of the
embryo by producing tension on the interconnected cell sheet (Figure 2.13I). The em-
bryo everts through this hole and then
closes it up. About a day after this is done,
the juvenile Volvox are enzymatically released from the parent and swim away.
What happens to the somatic cells of the
“parent” Volvox now that its young have
“left home”? Having produced offspring
and being incapable of further reproduction, these somatic cells die. Actually, these
cells commit suicide, synthesizing a set of
proteins that cause the death and dissolution of the cells that make them
(Pommerville and Kochert 1982). Moreover,
in death, the cells release for the use of others, including their own offspring, all the
nutrients that they had stored during life.
“Thus emerges,” notes David Kirk, “one of
the great themes of life on planet Earth:
‘Some die that others may live.’”
In V. carteri, a specific gene, somatic
regulator A, or regA, plays a central role in
regulating cell death (Kirk 1988, 2001a).
This gene is expressed only in somatic cells,
Figure 2.12
Asexual reproduction in V. carteri. (A) When reproductive cells (gonidia) are mature, they enter a
cleavage-like stage of embryonic development to produce juveniles within the adult. Through a series of cell movements resembling gastrulation, the embryonic Volvox invert and are eventually released from the parent. The somatic cells of the parent, lacking the gonidia, undergo senescence and
die, while the juvenile Volvox mature. The entire asexual cycle takes 2 days. (B) Young adult spheres
of Volvox carteri being released from parent to become free-swimming individuals. Once the progeny
leave, the parent undergoes programmed cell death. (A after Kirk 1988; B from Kirk 2001b.)
(A)
(B)
Expansion of
both adult and
juveniles
Embryogenesis
Adult with
juveniles
Adult with
mature gonidia
Maturation
of gonidia
Continued
expansion of
juveniles
fpo
Release
of juveniles
Continued
expansion of
extracellular
matrix
Death of parental
somatic cells
2ND PASS PAGE PROOFS
Life cycles and the evolution of developmental patterns
(A)
(B)
(C)
(D)
(E)
(F)
(G)
(H)
Figure 2.13
Inversion of embryos of V. carteri. A–D are scanning electron micrographs of whole embryos.
E–H are sagittal sections through the center of the embryo, visualized by differential interference
microscopy. Before inversion, the embryo is a hollow sphere of connected cells with the new gonidia on the outside. When the “bottle cells” change their shape, a hole (the phialopore) opens at the
apex of the embryo (A, B, E, F). Cells then curl around and rejoin at the bottom (C, D, G, H). The
new gonidia are now inside. (I)“Bottle cells” near the opening of the phialopore in a V. carteri embryo. These cells remain tightly interconnected through cytoplasmic bridges near their elongated
apices, thereby creating the tension that causes the curvature of the interconnected cell sheet.
(From Kirk et al. 1982; photograph courtesy of D. Kirk.)
and it prevents their expressing gonidial
genes. In laboratory strains possessing regulatory mutations of this gene, somatic
cells begin expressing regA, abandon their
suicidal ways, gain the ability to reproduce
asexually, and become potentially immortal (Figure 2.14). The fact that such mutants
have never been found in nature indicates
that cell death most likely plays an important role in the survival of V. carteri under
natural conditions.
the animal kingdom and all for the sake of
sex.’ And he asked: ‘Is it worth it?’
For Volvox carteri, it most assuredly is
worth it. V. carteri lives in shallow temporary ponds that fill with spring rains but dry
out in the heat of late summer. Between
(A)
37
(I)
those times, V. carteri swims about, reproducing asexually. These asexual volvoxes
will die in minutes once the pond dries up.
V. carteri is able to survive by turning sexual shortly before the pond disappears, producing dormant zygotes that survive the
heat and drought of late summer and the
cold of winter. When rain fills the pond in
spring, the zygotes break their dormancy
and hatch out a new generation of individuals that reproduce asexually until the
pond is about to dry up once more.
How do these simple organisms predict
the coming of adverse conditions so accurately that they can produce a sexual generation in the nick of time, year after year?
The stimulus for switching from the asexual to the sexual mode of reproduction in V.
(B)
Enter Sex
Although V. carteri reproduces asexually
much of the time, in nature it reproduces
sexually once each year. When it does, one
generation of individuals passes away and
a new and genetically different generation is produced. The naturalist Joseph
Wood Krutch (1956, pp. 28–29) put it more
poetically:
The amoeba and the paramecium are potentially immortal. … But for Volvox,
death seems to be as inevitable as it is in a
mouse or in a man. Volvox must die as
Leeuwenhoek saw it die because it had children and is no longer needed. When its
time comes it drops quietly to the bottom
and joins its ancestors. As Hegner, the Johns
Hopkins zoologist, once wrote, ‘This is the
first advent of inevitable natural death in
Figure 2.14
Mutation of a single gene (somatic regenerator A) abolishes programmed cell death in V. carteri.
(A) A newly hatched Volvox carrying this mutation is indistinguishable from the wild-type spheroid. (B) Shortly before the time when the somatic cells of wild-type spheroids begin to die, the somatic cells of this mutant redifferentiate as gonidia (B). Eventually, every cell of the mutant will
divide to regenerate a new spheroid that will repeat this potentially immortal developmental cycle.
(Photographs courtesy of D. Kirk.)
38
2ND PASS PAGE PROOFS
Chapter 2
carteri is known to be a 30-kDa sexual inducer protein. This protein is so powerful
that concentrations as low as 6 × 10–17 M
cause gonidia to undergo a modified pattern of embryonic development that results in the production of eggs or sperm,
depending on the genetic sex of the individual (Sumper et al. 1993). The sperm are
released and swim to a female, where they
fertilize eggs to produce dormant zygotes
(Figure 2.15). The sexual inducer protein is
able to work at such remarkably low concentrations by causing slight modifications
of the extracellular matrix. These modifications appear to signal the transcription of a
whole battery of genes that form the gametes (Sumper et al. 1993; Hallmann et al.
2001).
What is the source of this sexual inducer protein? Kirk and Kirk (1986) discovered
that the sexual cycle could be initiated by
heating dishes of V. carteri to temperatures
that might be expected in a shallow pond
in late summer. When this was done, the
somatic cells of the asexual volvoxes produced the sexual inducer protein. Since the
amount of sexual inducer protein secreted
by one individual is sufficient to initiate
sexual development in over 500 million
asexual volvoxes, a single inducing volvox
can convert an entire pond to sexuality.
This discovery explained an observation
made over 90 years ago that “in the full
blaze of Nebraska sunlight, Volvox is able
to appear, multiply, and riot in sexual re-
production in pools of rainwater of scarcely a fortnight’s duration” (Powers 1908).
Thus, in temporary ponds formed by spring
rains and dried up by summer’s heat,
Volvox has found a means of survival: it
uses that heat to induce the formation of
sexual individuals whose mating produces
zygotes capable of surviving conditions
that kill the adult organism. We see, too,
that development is critically linked to the
ecosystem in which the organism has
adapted to survive.
Figure 2.15
Sexual reproduction in V. carteri. Males and females are indistinguishable in their asexual phase.
When the sexual inducer protein is present, the gonidia of both mating types undergo a modified
embryogenesis that leads to the formation of eggs in the females and sperm in the males. When
the gametes are mature, sperm packets (containing 64 or 128 sperm each) are released and swim
to the females. Upon reaching a female, the sperm packet breaks up into individual sperm, which
can fertilize the eggs. The resulting dormant zygote has tough cell walls that can resist drying,
heat, and cold. When spring rains cause the zygote to germinate, it undergoes meiosis to produce
haploid males and females that reproduce asexually until heat induces the sexual cycle again.
Sperm packets
Sexual development
of gonidia
Sexual
inducer
Sperm
Asexual male
Gonidium
Modified embryonic
development of
gonidia resulting in
gamete production
Sexual male
Ova
Sexual
Zygotes
inducer
Egg
Asexual female
Sexual female
Meiosis and germination
Although all the volvocaceans, like their unicellular relative Chlamydomonas, reproduce predominantly by asexual
means, they are also capable of sexual reproduction, which involves the production and fusion of haploid gametes. In many
species of Chlamydomonas, including the one illustrated in
Figure 2.10, sexual reproduction is isogamous (“the same gametes”), since the haploid gametes that meet are similar in
size, structure, and motility. However, in other species of
Chlamydomonas—as well as many species of colonial volvocaceans—swimming gametes of very different sizes are produced by the different mating types. This pattern is called heterogamy (“different gametes”). But the larger volvocaceans
have evolved a specialized form of heterogamy called oogamy,
which involves the production of large, relatively immotile
eggs by one mating type and small, motile sperm by the other
(see Sidelights & Speculations). Here we see one type of gamete specialized for the retention of nutritional and developmental resources and the other type of gamete specialized for
the transport of nuclei. Thus, the volvocaceans include the
simplest organisms that have distinguishable male and female
members of the species and that have distinct developmental
pathways for the production of eggs or sperm.
In all volvocaceans, the fertilization reaction resembles
that of Chlamydomonas in that it results in the production of
a dormant diploid zygote that is capable of surviving harsh
environmental conditions. When conditions allow the zygote
2ND PASS PAGE PROOFS
to germinate, it first undergoes meiosis to produce haploid
offspring of the two different mating types in equal numbers.
Differentiation and morphogenesis in
Dictyostelium: Cell adhesion
Another type of multicellular organization derived from unicellular organisms is
found in Dictyostelium discoideum.* The life cycle of this fascinating organism is illustrated in Figure 2.16. In its asexual
cycle, solitary haploid amoebae (called myxamoebae or “social
amoebae” to distinguish them from amoeba species that always remain solitary) live on decaying logs, eating bacteria
and reproducing by binary fission. When they have exhausted
their food supply, tens of thousands of these myxamoebae
join together to form moving streams of cells that converge at
a central point. Here they pile atop one another to produce a
conical mound called a tight aggregate. Subsequently, a tip
arises at the top of this mound, and the tight aggregate bends
THE LIFE CYCLE OF DICTYOSTELIUM.
Life cycles and the evolution of developmental patterns
over to produce the migrating slug (with the tip at the front).
The slug (often given the more dignified title of pseudoplasmodium or grex) is usually 2–4 mm long and is encased in a
slimy sheath. The grex begins to migrate (if the environment
is dark and moist) with its anterior tip slightly raised. When it
reaches an illuminated area, migration ceases, and the culmination stages of the life cycle take place as the grex differentiates into a fruiting body composed of spore cells and a stalk.
The anterior cells, representing 15–20% of the entire cellular
population, form the tubed stalk. This process begins as some
of the central anterior cells, the prestalk cells, begin secreting
an extracellular cellulose coat and extending a tube through
the grex. As the prestalk cells differentiate, they form vacuoles
and enlarge, lifting up the mass of prespore cells that made
up the posterior four-fifths of the grex (Jermyn and Williams
1991). The stalk cells die, but the prespore cells, elevated
above the stalk, become spore cells. These spore cells disperse,
each one becoming a new myxamoeba.
WEBSITE 2.5 Slime mold life cycle. Check out this website to see the digitized videos from which the photographs
in Figure 2.16 were made.
*Though colloquially called a “cellular slime mold,” Dictyostelium is not a
mold, nor is it consistently slimy. It is perhaps best to think of Dictyostelium
as a social amoeba.
Low RES
fpo
Low RES
fpo
Slug (pseudoplasmodium;
grex)
15 hours
16 hours
17 hours
14 hours
20 hours
MIGRATION
Figure 2.16
Life cycle of Dictyostelium discoideum. Haploid spores give rise
to myxamoebae, which can reproduce asexually to form more haploid myxamoebae. As the food
supply diminishes, aggregation occurs at central points, and a migrating pseudoplasmodium is
formed. Eventually it stops moving
and culminates in a fruiting body
that releases more spores. The
times refer to hours since nutrient
starvation began the developmental sequence. The prestalk cells
have been indicated in yellow.
Low RES
fpo
Low RES
12 hours
Spore
case
CULMINATION
Low RES
fpo
AGGREGATION
fpo
Stalk
Spores
9 hours
Low RES
fpo
23 hours
10 hours
Tight
aggregate
6 hours
24 hours
Loose
aggregate
Cell streams
39
Myxamoebae
Mature
fruiting
body
Low RES
fpo
40
Chapter 2
In addition to this asexual cycle, there is a possibility of
sex for Dictyostelium. Two myxamoebae can fuse to create a
giant cell, which digests all the other cells of the aggregate.
When it has eaten all its neighbors, it encysts itself in a thick
wall and undergoes meiotic and mitotic divisions; eventually,
new myxamoebae are liberated.
Dictyostelium has been a wonderful experimental organism for developmental biologists because initially identical
cells differentiate into two alternative cell types—spore and
stalk. It is also an organism wherein individual cells come together to form a cohesive structure composed of differentiated cell types, a process akin to tissue formation in more
complex organisms. The aggregation of thousands of myxamoebae into a single organism is an incredible feat of organization that invites experimentation to answer questions about
the mechanisms involved.
VADE MECUM Slime mold life cycle. The life cycle of
Dictyostelium—the remarkable aggregation of myxamoebae, the migration of the slug, and the truly awesome culmination of the stalk and fruiting body—can best be
viewed through movies. The Slime Mold segment in Vade
Mecum contains a remarkable series of videos.
[Click on Slime Mold]
The first of these
questions is, What causes the myxamoebae to aggregate?
Time-lapse videomicroscopy has shown that no directed
movement occurs during the first 4–5 hours following nutrient starvation. During the next 5 hours, however, the cells can
be seen moving at about 20 mm/min for 100 seconds. This
movement ceases for about 4 minutes, then resumes. Although the movement is directed toward a central point, it is
not a simple radial movement. Rather, cells join with one another to form streams; the streams converge into larger
streams, and eventually all streams merge at the center. Bonner (1947) and Shaffer (1953) showed that this movement is a
result of chemotaxis: the cells are guided to aggregation centers by a soluble substance. This substance was later identified
as cyclic adenosine 3′5′-monophosphate (cAMP) (Konijn et
al. 1967; Bonner et al. 1969), the chemical structure of which
is shown in Figure 2.17A.
Aggregation is initiated as each of the myxamoebae begins to synthesize cAMP. There are no dominant cells that
begin the secretion or control the others. Rather, the sites of
aggregation are determined by the distribution of the myxamoebae (Keller and Segal 1970; Tyson and Murray 1989).
Neighboring cells respond to cAMP in two ways: they initiate
a movement toward the cAMP pulse, and they release cAMP
of their own (Robertson et al. 1972; Shaffer 1975). After this
happens, the cell is unresponsive to further cAMP pulses for
several minutes. The result is a rotating spiral wave of cAMP
that is propagated throughout the population of cells (Figure
2.17B–D). As each wave arrives, the cells take another step toward the center.*
AGGREGATION OF DICTYOSTELIUM CELLS.
2ND PASS PAGE PROOFS
The differentiation of individual myxamoebae into either
stalk (somatic) or spore (reproductive) cells is a complex matter. Raper (1940) and Bonner (1957) demonstrated that the
anterior cells normally become stalk, while the remaining,
posterior cells are usually destined to form spores. However,
surgically removing the anterior part of a slug does not abolish its ability to form a stalk. Rather, the cells that now find
themselves at the anterior end (and which originally had been
destined to produce spores) now form the stalk (Raper 1940).
Somehow a decision is made so that whichever cells are anterior become stalk cells and whichever are posterior become
spores. This ability of cells to change their developmental fates
according to their location within the whole organism and
thereby compensate for missing parts is called regulation. We
will see this phenomenon in many embryos, including those
of mammals.
CELL ADHESION MOLECULES IN DICTYOSTELIUM. How do individual cells stick together to form a cohesive organism? This
problem is the same one that embryonic cells face, and the solution that evolved in the protists is the same one used by embryos: developmentally regulated cell adhesion molecules.
While growing mitotically on bacteria, Dictyostelium cells
do not adhere to one another. However, once cell division
stops, the cells become increasingly adhesive, reaching a
plateau of maximum adhesiveness about 8 hours after starvation. The initial cell-cell adhesion is mediated by a 24-kilodalton glycoprotein (gp24) that is absent in myxamoebae but appears shortly after mitotic division ceases (Figure 2.18A;
Knecht et al. 1987; Wong et al. 1996). This protein is synthesized from newly transcribed mRNA and becomes localized in
the cell membranes of the myxamoebae. If myxamoebae are
treated with antibodies that bind to and mask this protein, the
cells will not stick to one another, and all subsequent development ceases.
Once this initial aggregation has occurred, it is stabilized
by a second cell adhesion molecule. This 80-kDa glycoprotein
(gp80) is also synthesized during the aggregation phase. If it is
defective or absent in the cells, small slugs will form, and their
fruiting bodies will be only about one-third the normal size.
Thus, the second cell adhesion system seems to be needed for
*The biochemistry of this reaction involves a receptor that binds cAMP. When
this binding occurs, specific gene transcription takes place, motility toward the
source of the cAMP is initiated, and adenyl cyclase enzymes (which synthesize
cAMP from ATP) are activated. The newly formed cAMP activates the cell’s
own receptors as well as those of its neighbors. The cells in the area remain insensitive to new waves of cAMP until the bound cAMP is removed from the
receptors by another cell surface enzyme, phosphodiesterase (Johnson et al.
1989). The mathematics of such oscillation reactions predict that the diffusion
of cAMP should initially be circular. However, as cAMP interacts with the cells
that receive and propagate the signal, the cells that receive the front part of the
wave begin to migrate at a different rate than the cells behind them (see
Nanjundiah 1997 1998). The result is the rotating spiral of cAMP and migration seen in Figure 2.17. Interestingly, the same mathematical formulas predict
the behavior of certain chemical reactions and the formation of new stars in
rotating spiral galaxies (Tyson and Murray 1989).
2ND PASS PAGE PROOFS
(A)
Adenine
O
CH2 O
5'
H
H
O
P
OH
(B)
3'
O
H
H
OH
cAMP
(C)
(D)
Figure 2.17
Chemotaxis of Dictyostelium myxamoebae is a result of spiral waves of
cAMP. (A) Chemical structure of cAMP. (B) Visualization of several
cAMP “waves.” Central cells secrete cAMP at regular intervals, and each
pulse diffuses outward as a concentric wave. The waves were charted by
saturating filter paper with radioactive cAMP and placing it on an aggregating colony. The cAMP from the secreting cells dilutes the radioactive
cAMP. When the radioactivity on the paper is recorded (by placing it over X-ray film), the regions of high cAMP concentration in the culture appear lighter than those of low cAMP concentration. (C) Spiral waves of myxamoebae moving toward the initial source of cAMP.
Because moving and nonmoving cells scatter light differently, the photograph reflects cell movement. The bright bands are composed of
elongated migrating cells; the dark bands are cells that have stopped moving and have rounded up. As cells form streams, the spiral of movement can still be seen moving toward the center. (D) Computer simulation of cAMP wave spreading across migrating Dictyostelium cells.
The model takes into account the reception and release of cAMP, and changes in cell density due to the movement of the cells. The cAMP
wave is plotted in dark blue. The population of amoebae goes from green (low) to red (high). Compare with the actual culture shown in (C).
(B from Tomchick and Devreotes 1981; C from Siegert and Weijer 1989; D from Dallon and Othmer 1997.)
(A)
(B)
Figure 2.18
The three cell adhesion molecules of Dictyostelium. (A) Dictyostelium cells synthesize an adhesive 24-kDa glycoprotein (gp24)
shortly after nutrient starvation. These Dictyostelium cells were
stained with a fluorescently labeled (green) antibody that binds to
gp24 and were then observed under ultraviolet light. This protein is
not seen on myxamoebae that have just stopped dividing. However,
(C)
as shown here—10 hours after cell division has ceased—individual
myxamoebae have this protein in their cell membranes and are capable of adhering to one another. (B) The gp80 protein, stained by
specific antibodies (green), is present at the cell membranes of
streaming amoebae. (C) The gp150 protein (green) is present in the
cells of the migrating grex (cross-sectioned). Photographs are not at
the same magnification. (Photographs courtesy of W. Loomis.)
42
2ND PASS PAGE PROOFS
Chapter 2
Sidelights
Speculations
Rules of Evidence I
B
iology, like any other science, does
not deal with Facts, but with evidence. Several types of evidence will
be presented in this book, and they are not
equivalent in strength. As an example, we
will use the analysis of cell adhesion in
Dictyostelium.
The first, and weakest, type of evidence
is correlative evidence. Here, correlations
are observed between two or more events,
and there is an inference that one event
causes the other. Correlative evidence provides a starting point for investigations,
but one cannot say with certainty that one
event causes the other based solely on correlations. As we have seen, fluorescently
labeled antibodies to a certain 24-kDa glycoprotein (gp24) do not bind to dividing
myxamoebae, but they do find this protein
in myxamoeba cell membranes soon after
the cells stop dividing and become competent to aggregate (see Figure 2.18A). Thus,
there is a correlation between the presence
of this cell membrane glycoprotein and the
ability to aggregate.
Although one might infer that the synthesis of gp24 causes the adhesion of the
cells, it is also possible that cell adhesion
causes the cells to synthesize this glycoprotein, or that cell adhesion and the synthesis
of the glycoprotein are separate events initiated by the same underlying cause. The
simultaneous occurrence of the two events
could even be coincidental, the events having no relationship to each other.*
How, then, does one get beyond mere
correlation? In the study of cell adhesion in
*In a tongue-in-cheek letter spoofing such correlative inferences, Sies (1988) demonstrated a remarkably good correlation between the number of
storks seen in West Germany from 1965 to 1980
and the number of babies born during those same
years.
Dictyostelium, the next step was to use the
antibodies that bound to gp24 to block the
adhesion of myxamoebae. Using a technique pioneered by Gerisch’s laboratory
(Beug et al. 1970), Knecht and co-workers
(1987) isolated the antibodies’ antigenbinding sites (the portions of the antibody
molecule that actually recognize the antigen). This step was necessary because the
whole antibody molecule contains two
antigen-binding sites and would therefore
artificially crosslink and agglutinate the
myxamoebae. When these antigen-binding
fragments (called Fab fragments) were
added to aggregation-competent cells, the
cells could not aggregate. The fragments
inhibited the cells’ adhesion, presumably
by binding to gp24 and blocking its function. This type of evidence is called loss-offunction evidence.
While stronger than correlative evidence, loss-of-function evidence still does
not make other inferences impossible. For
instance, perhaps the antibody fragments
kill the cells outright (as might have been
the case if gp24 were a critical transport
channel); this would also stop the cells
from adhering. Or perhaps gp24 has nothing to do with adhesion itself, but is necessary for the real adhesive molecule to function (perhaps, for example, it stabilizes
membrane proteins in general). In this
case, blocking the glycoprotein would similarly cause the inhibition of cell aggregation. Thus, loss-of-function evidence must
be bolstered by many controls demonstrating that the agents causing the loss of
function specifically knock out the particular function and nothing else.
The strongest type of evidence is gainof-function evidence. Here, the initiation
of the first event causes the second event
to happen even under circumstances
where neither event usually occurs. For in-
retaining a large enough number of cells to form large fruiting
bodies (Müller and Gerisch 1978; Loomis 1988). During late
aggregation, the levels of gp80 decrease, and its role is taken
over by a third cell adhesion protein, a 150-kDa protein
(gp150) whose synthesis becomes apparent just prior to aggregation and which stays on the cell surface during grex migration (Wang et al 2000; Figure 2.18). If Dictyostelium cells
lack functional genes for gp150, development is arrested at the
loose aggregate stage, and the prespore and prestalk cells fail
to sort out into their respective regions.* Thus, Dictyostelium
stance, da Silva and Klein (1990) and Faix
and co-workers (1990) obtained such evidence to show that the 80-kDa glycoprotein (gp80) is an adhesive molecule in
Dictyostelium. They isolated the gp80 gene
and modified it in a way that would cause
it to be expressed all the time. They then
placed it back into well-fed, dividing myxamoebae, which do not usually express this
protein and are not usually able to adhere
to one another. The presence of this protein on the membranes of these dividing
cells was confirmed by antibody labeling.
Moreover, the treated cells now adhered to
one another even in the proliferative stage
(when they normally do not). Thus, they
had gained adhesive function solely by expressing this particular glycoprotein on
their cell surfaces. Similar experiments
have recently been performed on mammalian cells to demonstrate the presence
of particular cell adhesion molecules in the
developing embryo. Such gain-of-function
evidence is more convincing than other
types of evidence.
This “find it; lose it; move it” progression of evidence is at the core of nearly all
studies of developmental mechanisms
(Adams 2000). Sometimes we find the entire progression in a single paper, but more
often, as the case above illustrates, the evidence comes from many laboratories. Such
evidence must be taken together. “Every
scientist,” writes Fleck (1979), “knows just
how little a single experiment can prove or
convince. To establish proof, an entire system of experiments and controls is needed.” Science is a communal endeavor, and
it is doubtful that any great discovery is the
achievement of a single experiment, or of
any individual. Correlative, loss-of-function, and gain-of-function evidence must
consistently support each other to establish
and solidify a conclusion.
has evolved three developmentally regulated systems of cellcell adhesion that are necessary for the morphogenesis of individual cells into a coherent organism. As we will see in sub*The gp150 cell adhesion protein of Dictyostelium may be critical for the sorting out of the prespore and prestalk cells in the grex. This protein is first expressed in prestalk cells and leads to their sorting out from prespore cells. A
few hours later it is also expressed in prespore cells but at a lower level. If the
protein is absent, no sorting out occurs. Thus, it appears that the temporal difference in expression and the levels of expression of this protein in the cell
types allows them to sort out (W. Loomis, personal communication).
2ND PASS PAGE PROOFS
sequent chapters, metazoan cells also use cell adhesion molecules to form the tissues and organs of the embryo.
Dictyostelium is a “part-time multicellular organism” that
does not form many cell types (Kay et al. 1989), and the more
complex multicellular organisms do not form by the aggregation of formerly independent cells. Nevertheless, many of the
principles of development demonstrated by this “simple” organism also appear in the embryos of more complex phyla
(see Loomis and Insall 1999). The ability of individual cells to
sense a chemical gradient (as in the myxamoeba’s response to
cAMP) is crucial for cell migration and morphogenesis during animal development. Moreover, the role of cell surface
proteins in cell cohesion is seen throughout the animal kingdom, and differentiation-inducing molecules are now being
isolated in metazoan organisms.
Differentiation in Dictyostelium
Differentiation into stalk cell or spore cell reflects another
major phenomenon of embryogenesis: the cell’s selection of a
developmental pathway. Cells often select a particular developmental fate when alternatives are available. A particular cell
in a vertebrate embryo, for instance, can become either an
epidermal skin cell or a neuron. In Dictyostelium, we see a
simple dichotomous decision, because only two cell types are
possible. How is it that a given cell becomes a stalk cell or a
spore cell? There appears to be a progressive commitment to
Life cycles and the evolution of developmental patterns
one of the two alternative pathways (Figure 2.19). At first
there is a bias toward one path or another. Then, there is a labile specification, a time when the cell will normally become
either a spore cell or a stalk cell, but when it can still change its
fate if placed in a different position in the organism. The third
and fourth stages are a firm commitment to a specific fate, followed by the cell’s differentiation into a particular cell type, either a stalk cell or a spore cell.
BIAS. Although the details are not fully known, a cell’s fate appears to be regulated by both internal and external agents. Preaggregation myxamoebae are not all the same; they can differ
in several ways. The internal factors distinguishing individual
myxamoebae include nutritional status, cell size, cell cycle
phase at starvation, and intracellular calcium levels (Nanjundiah 1997; Azhar et al. 2001). Each of these factors can act to
bias the cell toward a prespore or a prestalk pathway. For instance, cells starved in the S and early G2 phases of the cell
cycle have relatively high levels of calcium and display a tendency to become stalk cells, while those starved in mid- or late
G2 have lower calcium levels and tend to become spore cells.
LABILE SPECIFICATION. Several external factors are also important in specifying cells as stalk or spore. Ammonia, a product
of protein degradation, stimulates prespore gene expression
and suppresses the expression of those genes that would lead
(C) Stable commitment
and differentation
(A) Bias
Amoebae
Upper cup
fpo
Spore head or
sorus (pre-spore
cells)
(B) Labile
commitment
Lower cup
Grex
Stalk
Pre-stalk
cells
Pre-spore
cells
Figure 2.19
Alternative cell fates in Dictyostelium discoideum. (A–C) Progressive
commitment of cells to become either spore or stalk cells. (A)
Myxamoebae may have biases toward stalk or spore formation due
to the stage of the cell cycle they were in when starved. (B) As the
grex migrates, most prestalk cells are in the anterior third of the
grex, while most of the posterior two-thirds are prespore cells.
Some prestalk cells are also seen in the posterior, and these cells will
contribute to the cups of the spore sac and to the basal disc at the
bottom of the stalk. The grex can still regulate, however, and if the
43
(D)
(E)
(F)
(G)
Pre-stalk
and stalk
cell strain
Pre-spore
and spore
cell strain
Basal disk
stalk-forming anterior is cut off, the anteriormost cells remaining
will convert from stem to stalk. (C) At culmination, the sporeforming cells are massed together in the spore sac. The stalk cells
form the cups of the spore sac, as well as the stalk and basal disc.
(D, E) Grex and culminant stained with dye that recognizes the extracellular matrix of the prestalk and stalk cells. (F, G) Grex and
culminant stained with a dye that recognizes the extracellular matrix of prespore and spore cells. (After Escalante and Vicente 2000.
Photographs courtesy of XXXX XXXXX.)
44
Chapter 2
the cell to become a stalk (Oyama and Blumberg 1986). Prestalk cells are able to form only when ammonia is depleted,
and this appears to occur by the diffusion of ammonia at the
raised anterior tip of the grex. Cyclic AMP may also function
to induce prespore cell formation (Ginsburg and Kimmel
1997). High concentrations of cAMP initiate the expression of
spore-specific mRNAs in aggregated myxamoebae. Moreover,
when slugs are placed in a medium containing an enzyme that
destroys extracellular cAMP, the prespore cells lose their differentiated characteristics (Figure 2.20; Schaap and van Driel
1985; Wang et al. 1988a,b). Thus, cAMP works both as an extracellular signal (for chemotaxis) and an intracellular signal
(to activate those genes responsible for spore formation).
In the pathway to stalk cells, calcium appears to play a
critical role. High calcium levels appear to push cells into the
prestalk pathway, and the percentage of stalk cells can be in-
(A)
2ND PASS PAGE PROOFS
creased by manipulating the slug to have higher calcium levels
(Cubitt et al. 1995; Jaffe 1997). A secreted chlorinated lipid,
DIF-1, also plays some role in making prestalk cells, and it
may induce those genes that make the stalk-specific extracellular matrix. These two factors may act synergistically to push
cells with high calcium levels into the prestalk pathway.
COMMITMENT AND DIFFERENTIATION. Two secreted proteins,
spore differentiation factors SDF1 and SDF2, appear to be important in the final differentiation of the prespore cells into
encapsulated spores (Anjard et al. 1998a,b). SDF1 is important in initiating culmination, while SDF2 seems to cause the
prespore cells (but not prestalk cells) to become spores. The
prespore cells appear to have a receptor that enables them to
respond to SDF2, while the prestalk cells lack this receptor
(Wang et al. 1999). The formation of stalk cells from prestalk
cells is similarly complicated and may involve several factors
working synergistically (Early 1999). The differentiation of
stalk cells appears to need a signal from the intracellular enzyme PKA, and at least one type of stalk cell is induced by the
DIF-1 lipid (Thompson and Kay 2000; Fukuzawa et al. 2001).
Developmental Patterns among the Metazoa
(B)
(C)
Figure 2.20
Chemicals controlling differentiation in Dictyostelium. (A, B) Effects
of placing a Dictyostelium grex into a medium containing enzymes
that destroy extracellular cAMP. (A) Control grex stained for the
presence of a prespore-specific protein (white regions). (B) Similar
grex stained after treatment with cAMP-degrading enzymes. No
prespore-specific product is seen. (C) Higher magnification of a
slug treated with DIF (in the absence of ammonia). The stain used
here binds to the cellulose wall of the stalk cells. (A, B from Wang
etal., 1988a; C from Wang and Schaap, 1989; courtesy of the authors.)
Since most of the remainder of this book concerns the development of metazoans—multicellular animals* that pass
through embryonic stages of development—we will present
an overview of their developmental patterns here. Figure 2.21
illustrates the major evolutionary trends of metazoan development. The most striking pattern is that life has not evolved
in a straight line; rather, there are several branching evolutionary paths. We can see that metazoans belong to one of three
major branches: diploblasts, protostomes, and deuterostomes.
The sponges (Porifera) develop in a manner so different
from that of any other animal group that some taxonomists
do not consider them metazoans at all, and call them “parazoans.” A sponge has three major types of somatic cells, but
one of these, the archeocyte, can differentiate into all the
other cell types in the body. Individual cells of a sponge
passed through a sieve can reaggregate to form new sponges.
Moreover, in some instances, such reaggregation is speciesspecific: if individual sponge cells from two different species
are mixed together, each of the sponges that re-forms contains
cells from only one species (Wilson 1907). In these cases, it is
thought that the motile archeocytes collect cells from their
own species and not from others (Turner 1978). Sponges contain no mesoderm, so the Porifera have no true organ systems, nor do they have a digestive tube, circulatory system,
*Plants undergo equally complex and fascinating patterns of embryonic and
postembryonic development. However, plant development differs significantly from that of animals, and the decision was made to focus this text on the development of animals. Readers who wish to discover some of the differences
are referred to Chapter 20, which provides an overview of plant life cycles and
the patterns of angiosperm (seed plant) development.
2ND PASS PAGE PROOFS
Life cycles and the evolution of developmental patterns
45
Vertebrata
Cephalochordata
(Amphioxus)
Anus from
blastopore
Urochordata
(ascidians)
DEUTEROSTOMES
Hemichordata
Echinodermata
Bilateral symmetry,
three germ layers
(Bilateria)
Arthropoda
Nematoda
Ecdysozoa
Priapulida
Molting
Annelida
Mouth from
blastopore
Radial symmetry,
two germ layers
(Radiata)
Spiral
cleavage
Brachiopoda
PROTOSTOMES
Sipunculida
Lophotrochozoa
(Spiralia)
Mollusca
Colonial
protists
True
multicellularity
Rotifera
Platyhelminthes
(flatworms)
Ctenophora
(comb jellies)
Cnidaria
(jellyfish)
Figure 2.21
Major evolutionary divergences in extant animals. Other models of evolutionary
relationships among the phyla are possible. This grouping of the Metazoa is based
on embryonic, morphological, and molecular criteria. (Based on J. R. Garey, personal communication.)
Porifera
(sponges)
DIPLOBLASTIC
ANIMALS
46
2ND PASS PAGE PROOFS
Chapter 2
nerves, or muscles. Thus, even though they pass through an
embryonic and a larval stage, sponges are very unlike most
metazoans (Fell 1997). However, sponges do share many features of development (including gene regulatory proteins and
signaling cascades) with all the other animal phyla, suggesting
that they share a common origin (Coutinho et al. 1998).
The diploblasts
Diploblastic animals are those that have ectoderm and endoderm, but no true mesoderm. The diploblasts include the
cnidarians (jellyfish and hydras) and the ctenophores (comb
jellies).
Cnidarians and ctenophores constitute the Radiata, so
called because they have radial symmetry, like that of a tube
or a wheel. In these animals, the mesoderm is rudimentary,
consisting of sparsely scattered cells in a gelatinous matrix.
Phyla in the deuterostome lineage include the chordates
and echinoderms. Although it may seem strange to classify
humans, fish, and frogs in the same group as starfish and sea
urchins, certain embryological features stress this kinship.
First, in deuterostomes (“mouth second”), the oral opening is
formed after the anal opening. Also, whereas protostomes
generally form their body cavities by hollowing out a solid
block of mesoderm (schizocoelous formation of the body
cavity), most deuterostomes form their body cavities from
mesodermal pouches extending from the gut (enterocoelous
formation of the body cavity). It should be mentioned that
there are many exceptions to these generalizations.
The evolution of organisms depends on inherited
changes in their development. One of the greatest evolutionary advances—the amniote egg—occurred among the
deuterostomes. This type of egg, exemplified by that of a
chicken (Figure 2.22), is thought to have originated in the amphibian ancestors of reptiles about 255 million years ago. The
amniote egg allowed vertebrates to roam on land, far from existing ponds. Whereas most amphibians must return to water
to lay their eggs, the amniote egg carries its own water and
food supplies. It is fertilized internally and contains yolk to
nourish the developing embryo. Moreover, the amniote egg
contains four sacs: the yolk sac, which stores nutritive pro-
Protostomes and deuterostomes
Most metazoans have bilateral symmetry and three germ layers. The evolution of the mesoderm enabled greater mobility
and larger bodies because it became the animal’s musculature
and circulatory system. The animals of these phyla are known
collectively as the Bilateria. All Bilateria are thought to have descended from a primitive type of flatworm. These flatworms
were the first to have a true mesoderm (although
it was not hollowed out to form a body cavity),
Embryo
and they may have resembled the larvae of certain
contemporary coelenterates.
O2
Amnion
CO2
Animals of the Bilataria are further classified
Heart
as either protostomes or deuterostomes. Protostomes (Greek, “mouth first”), which include the
Allantois
mollusc, arthropod, and worm phyla, are so called
because the mouth is formed first, at or near the
Amniotic
opening to the gut, which is produced during gascavity
trulation. The anus forms later at another location.
The coelom, or body cavity, of these animals forms
from the hollowing out of a previously solid cord
of mesodermal cells. There are two major branches
Vitelline vein
of the protostomes. The Ecdysozoa includes the
Vitelline artery
animals that molt their exterior skeletons.* The
major constituent of this group is Arthropoda, a
phylum containing the insects, arachnids, mites,
Chorion
Shell
crustaceans, and millipedes. The second major
Shell
group of protostomes is the Lophotrochozoa.
Yolk
membrane
sac
These animals are characterized by a common type
Yolk
of cleavage (spiral), a common larval form (the
Figure 2.22
trochophore), and a distinctive feeding apparatus
Diagram of the amniote egg of the chick, showing the membranes enfolding the
(the lophophore) found in some species. Lopho7-day chick embryo. The yolk is eventually surrounded by the yolk sac, which allows the entry of nutrients into the blood vessels. The chorion is derived in part
trochozoan phyla include the flatworms, bryfrom the ectoderm and extends from the embryo to the shell (where it will fuse
ozoans, annelids, and molluscs.
*The name Ecdysozoa is derived from the Greek ecdysis, “to
shed” or “to get clear of.” Asked to provide a more dignified job
description for a “stripper,” editor H. C. Mencken suggested
the term “ecdysiast.”
with the blood vessel-rich allantois. This chorioallantoic membrane will exchange oxygen and carbon dioxide and absorb calcium from the shell). The amnion provides the fluid medium in which the embryo grows, and the allantois
collects nitrogenous wastes that would be dangerous to the embryo. Eventually
the endoderm becomes the gut tube and encircles the yolk.
2ND PASS PAGE PROOFS
teins; the amnion, which contains the fluid bathing the embryo; the allantois, in which waste materials from embryonic
metabolism collect; and the chorion, which interacts with the
outside environment, selectively allowing materials to reach
the embryo.* The entire structure is encased in a shell that allows the diffusion of oxygen but is hard enough to protect the
embryo from environmental assaults and dehydration. A similar development of egg casings enabled arthropods to be the
first terrestrial invertebrates. Thus, the final crossing of the
boundary between water and land occurred with the modification of the earliest stage in development: the egg.
VADE MECUM The amniote egg. The egg of the chick, as
detailed in this sequence, is a beautiful and readily accessible example of the amniote egg—a remarkable adaptation
to terrestrial life. [Click on Chick-early]
*In mammals, the chorion is modified to form the embryonic portion of the
placenta—another example of the modification of development to produce
evolutionary change.
Life cycles and the evolution of developmental patterns
47
Embryology provides an endless assortment of fascinating animals and problems to study. In this text, we will use but
a small sample of them to illustrate the major principles of
animal development. This sample is an incredibly small collection. We are merely observing a small tide pool within our
reach, while the whole ocean of developmental phenomena
lies before us.
After a brief outline of the experimental and genetic approaches to developmental biology, we will investigate the
early stages of animal embryogenesis: fertilization, cleavage,
gastrulation, and the establishment of the body axes. Later
chapters will concentrate on the genetic and cellular mechanisms by which animal bodies are constructed. Although an
attempt has been made to survey the important variations
throughout the animal kingdom, a certain deuterostome
chauvinism may be apparent. (For a more comprehensive survey of the diversity of animal development across the phyla,
see Gilbert and Raunio 1997.)
Principles of Development: Life Cycles and Developmental Patterns
1. The life cycle can be considered a central unit in biology.
The adult form need not be paramount. In a sense, the life
cycle is the organism.
2. The basic life cycle consists of fertilization, cleavage, gastrulation, germ layer formation, organogenesis, metamorphosis, adulthood, and senescence.
3. Reproduction and sex are two separate processes that may
but do not necessarily occur together. Some organisms,
such as Volvox and Dictyostelium, exhibit both asexual reproduction and sexual reproduction.
4. Cleavage divides the zygote into numerous cells called
blastomeres.
5. In animal development, gastrulation rearranges the blastomeres and forms the three germ layers.
6. Organogenesis often involves interactions between germ
layers to produce distinct organs.
7. Germ cells are the precursors of the gametes. Gametogenesis forms the sperm and the eggs.
8. There are three main ways to provide nutrition to the developing embryo: (1) supply the embryo with yolk; (2) form
a larval feeding stage between the embryo and the adult; or
(3) create a placenta between the mother and the embryo.
9. Life cycles must be adapted to the nonliving environment
and interwoven with other life cycles.
10. Don’t regress your tail until you’ve formed your hindlimbs.
11. There are several types of evidence. Correlation between
phenomenon A and phenomenon B does not imply that A
causes B or that B causes A. Loss-of-function data (if A is
experimentally removed, B does not occur) suggests that A
causes B, but other explanations are possible. Gain-offunction data (if A happens where or when it does not
usually occur, then B also happens in this new time or
place) is most convincing.
12. Protostomes and deuterostomes represent two different
sets of variations on development. Protostomes form the
mouth first, while deuterostomes form their mouths later,
usually forming the anus first.
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